The present invention concerns an absorber body arranged to absorb radiation in a nuclear energy arrangement.
Such absorber bodies are already well known within nuclear energy technology. They are arranged to be positioned in control rods, by means of which the irradiation of nuclear fuel in the reactor core is controlled. The absorber bodies are thereby often formed by relatively small rods of for example boron carbide which are positioned in holes arranged in blades of said control rods. Normally a plurality of such absorber body rods is arranged end to end after each other in the respective holes in the blades of the control rod. The holes are then sealed such that the absorber bodies are hermetically enclosed in the control rods.
During the subsequent operation of the nuclear energy arrangement, it is the task of the absorber bodies to absorb neutron radiation which thereby exists. The usually used absorber body material, boron carbide, will thereby in the course of time expand when it is exposed to said radiation. The absorber body will thus expand both in axial and radial direction.
The radiation is not evenly distributed over the whole absorber body. For example the ends of the absorber body will be exposed to more radiation than the other parts of the absorber body. The reason for this is that the radiation, the neutrons, at the ends of the absorber body may impinge against the uncovered end surface of the absorber body and thus hit the end surface which thereby expands. Such an increased neutron flux against the absorber body is particularly manifested at the ends of the absorber bodies which are positioned at the ends of an array of a plurality of absorber bodies arranged after each other.
Furthermore, the intensity of the radiation is usually different on different sides of the absorber body. When the absorber body is formed by an elongated rod which thereby is exposed to an increased neutron flux against a part of its envelope surface, the rod will expand, i.e. swell up more along said part. The rod may thereby possibly be bent. The bend may be so large that the absorber rod abuts and applies pressure forces against the surrounding wall of the control rod. The control rod normally consists of steel with a tendency to stress corrosion. The bending of the absorber rod thus results in a risk of stress corrosion cracks in the wall of the control rod which surrounds the absorber rod.
Furthermore, this phenomenon is accentuated by the locally increased expansion at the ends of the absorber rods.
In absorber bodies a so-called self-shielding exists, i.e. that peripheral areas of the absorber body have such a radiation absorbing ability that only a reduced amount of radiation reaches into the central portion of the body. The centre of the absorber bodies will therefore be exposed to a smaller amount of radiation than their peripheral portions. Tensions will arise between the centre and the peripheral portions, since the latter are the subject of an increased expansion. In the worst case the tensions may become so large in the axial direction that the absorber rod-breaks. A solid rod would, based on experience, because of inner tensions thereby break into pieces which have a length which essentially corresponds to the diameter of the rod. Such a splitting of an absorber rod into a plurality of pieces is not wanted.
The purpose with the present invention is to obtain an absorber body which has such a shape that the above mentioned disadvantages are reduced or completely eliminated. The absorber body should also be simple and inexpensive to produce.
This purpose is achieved by an absorber body as initially defined, which is characterised in that it comprises at least one area with a locally reduced thickness. By providing the absorber body with such an area, a local expansion space is created in this area. The area may be designed such that the periphery of the absorber body locally may be allowed to expand just there, whereby tensions between the centre of the absorber body and its peripheral areas are reduced and a breakage of the absorber body caused by the presence of such tensions is avoided. By a suitable position of said area the tendency to bending of the body because of unevenly distributed radiation may also be reduced. The area with a locally reduced thickness may also have such a shape and extension that the absorber body tends to break at this area if it still is bent because of unevenly distributed radiation such that it applies pressure forces against surrounding control rod walls. The formation of stress corrosion cracks in the control rod wall caused in the above mentioned manner may thereby be reduced or eliminated.
According to a preferred embodiment, at least one area with a locally reduced thickness is defined by at least one recess, which is provided in an outer surface of the absorber body. By the provision of a recess, the area may be made very locally and a sharp and distinct expansion space may be formed. The recess thereby defines a space in which the outer portions of the absorber body may expand when they are exposed to a larger neutron flux than the centre of the absorber body. A recess is also simple to produce at the production of an absorber body.
According to a further preferred embodiment, the absorber body is elongated and said at least one recess extends transversely to the longitudinal direction of the absorber body. Since the recess extends in this manner, an axial expansion of the peripheral portion of the absorber body is made possible.
According to a further preferred embodiment, said at least one recess extends as a closed, annular loop around the circumference of the body. An expansion of the peripheral portion of the absorber body over the whole cross-section of the absorber body where the recess is provided is thereby made possible. Such a recess may in a simple manner be made so deep that the centre of the body in this area will be so weakened that the body will break there when it is exposed to bending forces, such as when it due to unevenly distributed radiation is bent such that it locally presses against the surrounding control rod wall. When the absorber body is a rod, it does not matter in which rotational position it is pushed into a receiving hole in the control rod blade, since a recess provided in this manner will allow an axial expansion of the peripheral portion of the body over the whole cross-section of the body.
According to a further preferred embodiment, said at least one recess covers approximately 2-15% of the total area of said outer surface, preferably 5-10%. For a typical absorber rod with a length of about 100 mm and a diameter of about 5 mm a sufficient expansion space is thereby created for the peripheral portions of the rod in axial direction, at the same time as the recess or recesses form such a small part of the area of the outer surface and of the volume of the body that no significant reduction of the radiation absorption ability of the body is the case.
According to a further preferred embodiment, the absorber body is essentially rod-shaped and said outer surface is its envelope surface.
The provision of one or more areas of the absorber body with a reduced thickness, i.e. with a reduced diameter, gives a particularly good solution to the problem which the invention concerns.
According to a further preferred embodiment, the ratio between the depth d of said recess and the thickness t of the absorber body in the area of the recess is d/txe2x89xa60.40, preferably d/txe2x89xa60.35. At a larger depth than these an unwanted weakness of the absorber body in the area where its thickness is reduced is in some cases obtained. The ratio d/t should, furthermore, be larger than 0.02 in order for the recess to have the functions which have been described above. One or more recesses with such a depth that d/t is less than approximately 0.02 may also be provided, but have then to fulfil a completely different task, primarily to function as positions for initiating breaks of the rod where it may be wanted that it breaks.
According to a further preferred embodiment, a plurality of recesses is arranged at a distance from each other as seen in the longitudinal direction of the body. It is thereby avoided that tensions arise between the peripheral portions of the body and its centre due to the larger amount of radiation and the larger expansion to which the peripheral portion of the absorber body is exposed relative to its centre. The width of the recesses are advantageously adjusted to the distance between the same, such that the total width which the recesses together define forms approximately 2-15%, preferably 5-10% of the length of the absorber body when it has the shape of a rod with an arbitrary cross-section.
According to a further embodiment, the absorber body is elongated and said area is located at at least one of its ends. By locating the area with a reduced thickness to at least one of the ends of such an elongated body, a space for expansion is created at that end. Since the absorber body is exposed to increased radiation at the end, and thus is the subject of an increased expansion in this area it is by this feature avoided that the body expands so much in radial direction at that end that it because of this will generate pressure forces against the surrounding control rod wall. The end area will also not be as critical in this regard when the absorber body due to the fact that it is subjected to unevenly distributed radiation is somewhat bent. The thickness reduction in this area is suitably adjusted such that the thickness of the body in this area never during the lifetime of the absorber body expands to an unproportional size in relation to the thickness of the remaining parts of the absorber body. By the provision of the area with a reduced thickness at said end it is in the case of absorber rods of boron carbide possible to avoid the use of a cylinder of hafnium which according to prior technology is positioned farthest out in the channel in which the absorber body/bodies are positioned with the purpose of suppressing the neutron flux against the outermost end of the absorber rod.
According to a further preferred embodiment, said area is defined by a bevel of the body at said end. Such a bevel gives, inter alia, a gradual adjustment of the thickness of the absorber body up to its absolute end, where it thus is most narrow and where the neutron flux is largest and where the body is subject to the largest expansion in radial direction.
The invention also concerns a control rod for a nuclear energy arrangement, which control rod is characterised in that it comprises at least one absorber body with the above mentioned features. Further advantages with and features of the invention will be clear from the remaining dependent claims and from the following description.